"Russian venture capitalist Yuri Milner, along with physicist Stephen Hawking, has announced a $100 million initiative for exploring Alpha Centauri, the star system nearest to our own. The new project is called Breakthrough Starshot, and the goal is to explore the technologies needed to create small, light-powered spacecraft capable of reaching Alpha Centauri in just 20 years. The project will be led by Pete Worden, the former director of NASA's AMES Research Center. Facebook founder and CEO Mark Zuckerberg will join Hawking and Milner on the project's board of directors." --Might be nice if we can do some analysis on the back of an envelope showing that this entire fantasy is a total crock with zero hope. Allegedly the spacecraft would mass about 1 gram, consist of a small but amazing instruments and brain ("cameras, photon thrusters, power supply, navigation and communications equipment"), plus a light sail up to a few meters in diameter but only 3-1000 atoms thick. Accelerated by light pressure from huge lasers (located on the moon? some high altitude location on Earth?) to reach speed 0.2c. The lasers would be 1 km diameter. Supposedly that whole acceleration to 0.2c will take only a "few minutes." Flock of several thousand of them launched simultaneously by rocket, but only 1 is accelerated to relativistic speed at a time. How does it communicate back to us after reaches alpha centauri? Little or no idea. http://www.sci-news.com/space/breakthrough-starshot-plan-alpha-centauri-0378... OK, here are a few calculations: Mass of sail 1000 angstrom thick, 10 meter^2 area, density of water: 1 gram. Kinetic energy of 1 gram at 0.2c: 1800 giga joules = 0.5 gigawatt hours. Can that much intelligence and sensory capability be done in 1gram? A large butterfly weighs a fraction of a gram and has eyes, navigation good enough to take it to one particular place in Mexico, brain, senses, thrust. So, maybe. Acceleration needed to reach 0.2c in 3 minutes: 34000 gees = 333000 meter/sec^2 which is about the same as a bullet experiences inside a pistol or rifle. For a 1 gram mass this is a force of 333 newtons. Tensile strengths: kevlar 3*10^8, steel 4*10^8, graphene 10^11 (all in pascals). So a 1000 angstrom thick film 1 meter wide with those strengths could pull 30-10000 newtons without breaking. Light pressure: A pressure of 100 newtons/meter^2 = 100 pascals on a reflective mirror, is achieved with beam power 30 gigawatt/meter^2. Lasers: wikipedia says "The AN/SEQ-3 Laser Weapon System or XN-1 LaWS is a directed-energy weapon developed by the United States Navy. The weapon was installed on USS Ponce for field testing in 2014... it generates (up to) 33 kWatt in testing..." a picture suggests the aperture diameter is about 2 feet, i.e. less than 1 meter. Allegedly the Navy claims it works well enough either as a weapon or in reverse as a telescope, that they want to keep it. So... looks like the laser beam intensity is a major problem for "starshot." Factor of a million short. I also have a hell of a big problem seeing how it could tell us anything from alpha centauri. There is some suspicion there are 1 or more planets orbiting one of the three alpha centauri stars, but so far that is unconfirmed. If we cannot even detect a freaking planet, then hard to detect communications from a tiny spaceprobe. If the power source was solar, then the probe will be within 1 AU of alpha centauri star for a total timespan of about 1.5 hours given its 0.2c speed. Even if it were capable of sending 1 bit per second (which seems to me incredibly optimistic) that'd be under 5 kbits total; I personally doubt (based on extrapolation of known bitrates from known spaceprobes), that it could send us even 1 bit ever. Actually perhaps the best try to send us 1 bit would be, if possible, to intentionally ram the spaceprobe into an asteroid so we could try to detect resulting explosion, which would be about 0.4 kilotons in nuclear weapons terminology. -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)
Light pressure: A pressure of 100 newtons/meter^2 = 100 pascals on a reflective mirror, is achieved with beam power 30 gigawatt/meter^2.
--Even if the mirrorfilm were 99.9% reflective, that'd mean said film would be absorbing 30 megawatt/meter^2 from this beam, which would have to be compensated by blackbody radiation for cooling, which would force the film to have a temperature of 4000 kelvin (calculation using stefan boltzmann law). This is even assuming a mirror film that reflective could at the same time be an excellent blackbody, which seems a contradiction. So that seems another damning obstacle. So yah, I think I've found enough problems in 15 minutes to kill the entire project that was claimed. Can I have my $100 million now?
Another issue with the "starshot" plan is the lightsail might tend to fold up, crumple, or otherwise deform into a poorly- or non-functioning shape. Is that problem defeatable? If we imagine a 1-dimensional lightsail in a 2D universe for simplicity, the lightsail is then a curve-segment festooned with 1 or more deadweights. If there are no deadweights at the curve endpoints, we clearly lose since the curve will just bend upward to vertical and nonfunctioning position. If there are two equal deadweights at the two curve endpoints, then they will experience vertical downforce from the gees, and also tension force from the lightsail-curve. The vector sum of these two forces will always have an inward component if the curve has finite |derivative|, hence such a lightsail will always "fold" as the two deadweights move inward to meet each other. So: Also lose. If the curve has vertical derivative at the endpoints (such as a semicircle) then there might be hope for a (perhaps unstable) equilibrium configuration. And instability (if any) perhaps could be active-controlled (albeit the enormous accelerations we are talking about might present some difficulty for controllers to handle). In a 3D universe with a rotationally symmetric (e.g. hemispherical-shaped) lightsail and a deadweights festooned along its rim, this again could (I think) be designed to enjoy equilibrium and actively controlled to overcome instabilities. Also there now is an additional option of adding spin causing centrifugal outward forces, which could try to compensate for the inward tensional forces. So I actually think this particular problem is not inherently insurmountable, albeit it is annoying to have so many deadweights round rim that all play active control games, rather than just one, like in the press description involving just ONE "chip" . But anyhow, bottom line is I think the whole "starshot" proposal probably is total no-hope hogwash; it is just that this particular problem is not the reason it is. -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)
On 4/16/2016 4:26 PM, Warren D Smith wrote:
Also there now is an additional option of adding spin causing centrifugal outward forces, which could try to compensate for the inward tensional forces.
Spinning the sail is the usual solution. It also gives the sail a stable orientation.
So I actually think this particular problem is not inherently insurmountable, albeit it is annoying to have so many deadweights round rim that all play active control games, rather than just one, like in the press description involving just ONE "chip" .
But anyhow, bottom line is I think the whole "starshot" proposal probably is total no-hope hogwash; it is just that this particular problem is not the reason it is.
I agree. Brent
-- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)
One reason planets are hard to see is the nearby star swamping the signal. If you choose a particular wavelength and signal encoding, you might get something detectable against this background. It's already been noted that our defense radars and broadcast TV signals are detectable at light-year distances. The proposed probes will be power starved, but they can use a very narrow beam signal. Another possible option is for the probes to communicate with each other. If an occasional heavier probe is included in the mix, it could handle reporting the results back to us. Another advantage of intercommunication is that one probe can tell its followers what to look at. Rich ------------------- Quoting Warren D Smith <warren.wds@gmail.com>:
But anyhow, bottom line is I think the whole "starshot" proposal probably is total no-hope hogwash; it is just that this particular problem is not the reason it is.
https://en.wikipedia.org/wiki/Starwisp looks to me (at first glance) slightly less insane than starshot. -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)
participants (3)
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Brent Meeker -
rcs＠xmission.com -
Warren D Smith